Regulation of neuronal mRNA splicing and Tau isoform ratio by ATXN3 through deubiquitylation of splicing factors

Abstract

33The ubiquitylation/deubiquitylation balance in cells is maintained by 34Deubiquitylating enzymes, including ATXN3. The precise role of this protein, 35 mutated in SCA3, remains elusive, as few substrates for its deubiquitylating 36 activity were identified. Therefore, we characterized the ubiquitome of 37 neuronal cells lacking ATXN3, and found altered polyubiquitylation in a large 38 proportion of proteins involved in RNA metabolism, including splicing factors. 39Using transcriptomic analysis and reporter minigenes we confirmed that 40 splicing was globally altered in these cells. Among the targets with altered 41 splicing was SRSF7 (9G8), a key regulator of MAPT (Tau) exon 10 splicing. 42Loss-of-function of ATXN3 led to a deregulation of MAPT exon 10 splicing 43 resulting in a decreased 4R/3R-Tau ratio. Similar alterations were found in the 44 brain of a SCA3 mouse and humans, pointing to a relevant role of this 45 mechanism in SCA3, and establishing a previously unsuspected link between 46 two key proteins involved in different neurodegenerative disorders. 47 48 Ataxin-3/ Neurodegeneration/ Serine-Arginine rich splicing factor 7 (SRSF7 or 49 9G8)/ Splicing/ Ubiquitylation 50 51 52 53 54 55 56 57 1,6 ). Impairment of the UPP has been connected to several neurodegenerative 75 diseases such as Alzheimer's (Reviewed in 7 ), Parkinson's (Reviewed in 8 ) 76 and Hungtington's (Reviewed in 9 ) diseases. Like most post-transcriptional 77 modifications, ubiquitylation is a reversible signal and is counterbalanced by 78 deubiquitylating (DUB) enzymes, which remove Ub units and play an 79 important role in the modulation of proteins by the proteasome as well as the 80 autophagic pathway 10-12 , and in Ub signaling in general. Several pieces of 81 evidence have suggested that Ub may also regulate splicing: (i) Ub and Ub-82 like proteins have been shown to copurify with splicing complexes 13,14 , (ii) 83 ubiquitylated splicing factors have been identified in proteomic screenings 15,16 84 and (iii) several functional domains related to Ub pathway have been 85 identified in important spliceosome proteins, such as Jab1/MPN domain of the 86 essential U5 SnRNP component Prp8 17-20 . Thus, the action of DUBs has a 87 major impact on the ubiquitylated proteome (also known as ubiquitome).88 Ataxin-3 (ATXN3) is a protein with DUB activity known to be involved in 89 Spinocerebellar ataxia type 3 (SCA3)/ Machado Joseph disease (MJD), a 90 neurodegenerative disorder of adult onset caused by the expansion of a 91 polyglutamine (polyQ) tract in this protein. Interestingly, in addition to being 92 involved in global protein quality control responses 21,22 , normal ATXN3 93 appears to participate in cell adhesion and in the organization of cytoskeleton 94 network 23-26 , to be involved in transcriptional regulation and DNA repair 27-29 , 95and to be required for neuronal differentiation 25 . Nevertheless, the relative 96 relevance of these functions and the physiological role of this highly 97 conserved but genetically non-essential prot...

Tau and neurofilament light‐chain as fluid biomarkers in spinocerebellar ataxia type 3

“…In later stages, these phenomena would be sequentially overridden by axonal loss (mirrored by a progressive rise in NfL) and neuronal cell loss. Finally, there could be a progressive dysregulation of tau expression in the SCA3 disease course, as shown in a preliminary study [19]. Nonetheless, for its implementation as a biomarker in SCA3, further studies in tau kinetics in CSF and blood will be required, as well as longitudinal clinical studies in large patient cohorts.…”

“…In a pilot study, cerebrospinal fluid (CSF) tau was elevated in patients with spinocerebellar ataxia type 2 (SCA2) and multiple system atrophy, cerebellar type (MSA‐c), compared to controls [18]. In SCA3, dysregulation of MAPT splicing with a decreased 4R/3R ratio has been observed [19].…”

Heat Shock Alters the Proteomic Profile of Equine Mesenchymal Stem Cells

“…Proteins SFSWAP, SRSF7, and DDX17 are involved in the formation of mature mRNAs through the control of mRNA transcription and splicing. In particular, SFSWAP and SRSF7 alter the spliceosome by repressing the splicing of exon 10 of the Tau protein [60,61]. Tau has been implicated in the pathogenesis of neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease [62].…”

“…The alternative splicing of Tau exon 10 causes a dysregulation in the balance of Tau isoforms, which has been shown to be correlated with the development of these neurodegenerative diseases [63]. SRSF7, serine/argininerich splicing factor 7, also has been associated with the ubiquitin proteasome pathway through which the Tau protein balance may be affected [61]. However, SFSWAP (splicing factor, suppressor of the white-apricot homolog), which is also known in humans as SRSF8, was identified through homology to other SRSF proteins, so the mechanism is less well understood than SRSF7 [64].…”